1. Field of the Invention
The present invention relates to an alternator in which an alternating voltage is generated in a stator by rotation of a rotor.
2. Description of the Related Art
FIG. 9 is a cross-section of a conventional automotive alternator (hereinafter simply "alternator"), and FIG. 10 is a perspective view of the rotor in FIG. 9, FIG. 11 is a front elevation of the stator core in FIG. 9, and FIG. 12 is an electrical circuit diagram of the alternator in FIG. 9.
The alternator includes: a case composed of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 5 rotably disposed in the case having a pulley 4 secured to a first end thereof; a Lundell-type rotor 6 secured to the shaft 5; fans 7 secured to both axial ends of the rotor 6; a stator 8 secured to the inner wall of the case 3; slip rings 9 secured to a second end of the shaft 5 for supplying electric current to the rotor 6; brushes 10 sliding in contact with the slip rings 9; brush holders 11 accommodating the brushes 10; first and second rectifiers 12a and 12b in electrical contact with the stator 8 for converting alternating current generated in the stator 8 into direct current; a heat sink 13 fitted over the brush holder 11; and a regulator 14 fastened to the heat sink 13 by adhesive for adjusting the magnitude of the alternating voltage generated in the stator 8.
The rotor 6 includes a field coil 15 for generating magnetic flux on passage of electric current, and a pole core 16 covering the field coil 15 in which magnetic poles are produced by the magnetic flux. The pole core 16 includes a first pole core assembly 17 and a second pole core assembly 18 which are mutually intermeshed. The first pole core assembly 17 and the second pole core assembly 18 are made of iron and have claw-shaped magnetic poles 19 and 20 at their ends. Spaces 21 are formed between adjacent claw-shaped magnetic poles 19 and 20 in order to prevent magnetic flus from leaking from between the claw-shaped magnetic poles 19 and 20, and also to function as cooling passages for cooling the field coil 15.
The stator 8 includes a stator core 22, and two three-phase stator coils 23a and 23b composed of wire wound onto the stator core with a phase difference of 30 electrical degrees. The stator core 22 shown in FIG. 11 is constructed by punching thin sheets of steel plate into an evenly-spaced comb shape and winding or laminating the comb-shaped plates into a ring shape. Slots 25 and teeth 24 extending in a radial direction are formed on an inner circumferential portion of the stator core 22.
This example has two sets of stator coil, namely the first three-phase stator coil 23a and the second three-phase coil 23b, and the rotor 6 has 12 poles with two three-phase portions corresponding to each pole, and there are therefore seventy-two slots 25 and teeth 24. The stator core 22 is annular, but is shown flattened out in FIG. 13, and the annular stator core 22 is formed with slots 25 at an even pitch of five mechanical degrees (360 degrees/72). Thus, because the seventy-two slots correspond evenly to the twelve poles, the slots 25 are formed at an even pitch of 30 electrical degrees.
The two star-connected three-phase stator coils 23a and 23b are disposed in the slots 25 with a phase difference of 30 electrical degrees from each other, and are electrically connected to the first rectifier 12a and the second rectifier 12b.
In an automotive alternator of the above construction, current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the field coil 15, generating magnetic flux, whereby the claw-shaped magnetic poles 19 of the first pole core assembly 17 are polarized with north-seeking (N) poles and the claw-shaped magnetic poles 20 of the second pole core assembly 18 are polarized with south-seeking (S) poles.
At the same time, the pulley 4 is rotated by an engine, and the rotor 6 rotates together with the shaft 5. Consequently, a rotating magnetic field is imparted to the stator coils 23a and 23b and electromotive force is generated. This alternating electromotive force is converted into direct current by means of the rectifiers 12a and 12b, its magnitude is regulated by the regulator 14, and the battery is recharged.
In this automotive alternator, there is one slot 25 in the stator core 22 for each set and phase of the stator coil 23 and for each pole of the rotor 6, and flux leakages formed between adjacent claw-shaped magnetic poles 19 and 20 of the rotor 6 rarely form within the same tooth 24, making flux leakage time in the teeth 24 short. Consequently, decreases in the amount of effective flux acting on the stator coil 23 due to flux leakages are rare and flux surges are reduced.
Moreover, the same technical content as the above automotive alternator is disclosed in Japanese Patent Laid-Open No. HEI 4-26345.
In a conventional automotive alternator, as shown in FIG. 3 which was prepared by the present inventors based on electromagnetic field analysis (A slot opening pitch of 24 degrees, for example, on the horizontal axis means that the electrical angle is at an even pitch of 30 degrees, and the physical angle is at an uneven pitch of 24 to 36 degrees. The vertical axis represents the ratio of harmonic frequencies to the fundamental frequency of the stator magnetomotive force), when the pitch of the slot openings is constant at an electrical angle of 30 degrees, the fifth and seventh spatial harmonic frequencies do not appear in the flux density waveform. However, one problem has been that when the eleventh and thirteenth spatial harmonic frequencies in the magnetomotive harmonic frequencies of the stator 8 are large and the eleventh or thirteenth harmonic frequencies are present in the magnetomotive harmonic frequencies of the rotor 6, suppression of magnetic flux surges is inadequate due to interference between the frequencies, making the suppression of variations in the generated voltage inadequate, and magnetic attraction arises between the claw-shaped magnetic poles 19 and 20 and the claw-shaped magnetic poles 19 and 20 or the rotor 6 vibrate, giving rise to a sound which is unpleasant to passengers in the vehicle.
Similarly, another problem has been that when the slot harmonic frequencies of the stator 8 are the eleventh and thirteenth spatial harmonic frequencies and the eleventh or thirteenth harmonic frequencies are present in the magnetomotive harmonic frequencies of the rotor 6, suppression of magnetic flux surges is inadequate due to interference between the frequencies, making the suppression of variations in the generated voltage inadequate, and magnetic attraction arises between the claw-shaped magnetic poles 19 and 20 or the rotor 6 and the stator 8, making the stator 8, the case 3, etc., resonate, or the claw-shaped magnetic poles 19 and 20 of the rotor 6 vibrate, giving rise to a sound which is unpleasant to passengers in the vehicle.